This invention relates generally to steam generating equipment and, more particularly, to steam separators for separating liquid water from gaseous steam in a steam-water mixture generated in such equipment.
Steam separators are well-known devices in the field of steam generating apparatus and equipment. Such separators receive a steam-water mixture generated in a boiler and function to separate liquid water components from gaseous steam components to ensure that only steam is delivered to a downstream device such as a steam turbine. Typically, steam separators are coupled to a boiler through steam-water risers. Flow is supplied to the steam separator in the form of a steam-water mixture. Steam separates from the water in the separator and is discharged through a steam outlet. Liquid water collected in the separator is returned to the boiler through one or more downcomers. "Primary" separation principally involves the removal of the bulk of the liquid water components from the steam. "Secondary" separation, or "steam scrubbing," involves removing most of the remaining water from the steam. Typically, both primary and secondary separation structures are included in a single steam separator.
Primary separation can be achieved mechanically in a number of ways, the simplest way being gravity separation. In gravity separation, the steam-water mixture is introduced into a closed vessel having an outlet at the top, and the inherent weight of the entrained liquid water causes the water to fall out of the steam flow. Although moderately effective, particularly at very low pressures and flow rates, gravity separation is largely ineffective at high pressures and flow rates. Gravity separation is also unsuitable in certain critical applications wherein it is important that liquid water be almost completely removed from the steam flow.
Primary separation can also be accomplished using various baffles or deflector plates that extend into the steam-water mixture flow and collect and channel the liquid water components away from the gaseous steam components. Such separation methods are far superior to simple gravity separation. In addition, they have the benefits of being relatively simple and inexpensive.
Still another way of achieving primary separation is through the use of centrifugal separators. In a typical centrifugal separator, the steam-water mixture is admitted tangentially into a cylindrical structure. Centrifugal force tends to drive the liquid components outwardly against the cylinder walls where they collect and drip down, while the gas components move toward the center where they collect and rise. Corrugated scrubbers near the upper end of the cylindrical structure further separate the liquid components from the gaseous ones. Although highly effective, centrifugal scrubbers are relatively complex and expensive.
In one form of primary mechanical separator, the effectiveness of centrifugal separation is largely combined with the simplicity and economy of baffle/deflector types of separators. In this form of separator, the steam-water mixture is introduced through the side of a cylindrical separator vessel below the level of the water ordinarily present in the vessel. An internal baffle near the vessel wall confines the steam-water mixture and causes it to flow upwardly along the interior side wall of the vessel. As the steam-water mixture breaks through the surface of the water, a curved deflector plate or baffle captures the flow and bends it back down toward the water surface. The resulting centripetal accelerations cause the liquid water components to accumulate along the curved baffle where they separate from the steam component and drip back down into the vessel. Although effective, such a separator requires that the steam pass through the "curtain" of water that drips off the end of the baffle. At high flow rates, this can reentrain some of the separated water, thereby reducing the overall effectiveness of the separator.